Ultrasound apparatus has been described that forms an, image of structures within a body of a patient in monochrome and superimposes colors thereon that are intended to indicate the velocity of blood flow, e.g., see U.S. patent application Ser. No. 748,531, filed June 25, 1985 by David Lipschutz and entitled "Flow Mapping Apparatus". Ideally, the monochrome image would only represent structures other than blood, and the colors would only appear where the blood is flowing. For various reasons, however, color appears in structural areas where there is no blood. This is caused in part by the fact that some structures, like heart walls, are in motion so as to produce velocity signals that cause color to appear in the image. Because the velocity of such motion is less than the velocity of blood flow, it is possible to reduce this effect by using what is known as a "wall filter"; but because of fundamental limitations, it cannot operate perfectly. Even without such wall motion, however, electrical noise may cause false indications of velocity because the signals from which the velocity is determined are weak. Furthermore, combining the monochrome image representing structure with colors representing velocity of flow by simple addition causes the colors to be changed by desaturation in a manner that is completely unrelated to velocity.